1. Field of the Invention
This invention relates to an optical probe, and more particularly to an optical probe having a tubular outer envelope and having a function of deflecting and scanning light emitted from the peripheral surface thereof in the direction of circumference or the axis of the outer envelope.
2. Description of the Related Art
As a method of obtaining a tomographic image of an object of measurement such as living tissue, it is proposed to obtain a tomographic image of the object by measuring OCT (optical coherence tomography) as disclosed in Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688. In the OCT measurement, a phenomenon that interference light is detected when the optical paths of the measuring light and the reflected light conform to the optical path of the reference light in length is used. That is, in this method, low coherent light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement is led to a multiplexing means. The reference light is led to the multiplexing means after its optical path length is changed in order to change the depth of measurement in the object. By the multiplexing means, the reflected light and the reference light are superposed one on another, and interference light due to the superposition is detected by, for instance, heterodyne detection.
In the above OCT system, a tomographic image is obtained by changing the optical path length of the reference light, thereby changing the measuring position (the depth of measurement) in the object. This technique is generally referred to as “TD-OCT (time domain OCT)”. More specifically, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 6(1994)-165784, an optical system which collects the reference light emitted from the optical fiber on a mirror is provided and the optical path length is adjusted by moving only the mirror in the direction of the beam axis of the reference light. Further, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 2003-139688, the reference light emitted from the optical fiber is turned to parallel light, the reference light in the form of parallel light is collected and caused to enter the optical fiber again by an optical path length adjusting lens, and the optical path length adjusting lens is moved back and forth in the direction of the beam axis of the reference light.
Whereas, as a system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an optical tomography system for obtaining an optical tomographic image by measurement of SD-OCT (spectral domain OCT). In the SD-OCT system, a tomographic image is formed without scanning in the direction of depth, by dividing broad band, low coherent light into measuring light and reference light by the use of a Michelson interferometer, projecting the measuring light onto the object and carrying out a Fourier analysis on each channeled spectrum obtained by decomposing the interference light of the reflected light, which returns at that time, and the reference light.
As another system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an optical tomography system for obtaining an optical tomographic image by measurement of SS-OCT (swept source OCT). In the SS-OCT system, the frequency of the laser beam emitted from the light source is swept to cause the reflected light and the reference light to interfere with each other at each wavelength, the intensity of the reflected light at the depth of the object is detected by Fourier-transforming the spectrum of the interference for the series of wavelength, and a tomographic image is formed by the use of the intensity of the reflected light at the depth of the object.
In the optical tomography system of each of the systems described above, a tomographic image along a certain surface of the object is generally obtained and for this purpose, it is necessary to at least one-dimensionally scan the measuring light beam in the object. As a means for effecting such a light scanning, there has been known, as disclosed in Japanese Unexamined Patent Publication No. 2002-005822 and International Patent Publication No. WO02/088684, an optical probe having a tubular outer envelope and having a function of deflecting and scanning light emitted from the peripheral surface thereof in the direction of circumference of the outer envelope. More specifically, the optical probe comprises a tubular outer envelope (sheath) closed at the leading end thereof, a shaft which is rotatable about an axis of rotation extending longitudinal direction of the outer envelope inside the outer envelope, a light guide means such as an optical fiber which is disposed inside the outer probe to extend along the shaft and is connected to the shaft at its leading end portion, a light deflecting means which is connected to the leading end portion of the light guide means and deflects light radiated from the leading end portion of the light guide means in a direction intersecting the axis of rotation of the shaft, and a collecting lens which converges light radiated from the light deflecting means outside the outer envelope, and deflects and scans light emitted from the light deflecting means in the direction of circumference of the outer envelope.
Further, as an optical probe similar to that described above, there has been known an optical probe which comprises a light guide means, a and a collecting lens similar to those described above in addition to a tubular outer envelope and a shaft movable in the longitudinal direction of the outer envelope inside the outer envelope, and causes light radiated from the light deflecting means to scan linearly in the direction of the movement in response to movement of the shaft in the longitudinal direction of the outer envelope.
When an optical tomographic image is to be obtained by the use of the optical probe described above, there has been a requirement that the focusing position (converging position) of the light beam which scans the object is changed according to the depth of the part to be observed. Further, there has been a requirement that the NA of the light beam which scans the object is changed according to the region to be observed and/or the resolution to be desired.
In the optical probe disclosed in Japanese Unexamined Patent Publication No. 2002-005822, the thickness of the outer envelope (sheath) is locally varied so that the position of the focusing position of the light beam can be changed.
However, in the above structure, though the focusing position is varied according to the eccentric position of the housing when the sheath is mounted thereon, how to control the eccentric position of the housing is not established yet. That is, though, when the focusing position happens to be conformed when the sheath is mounted on the housing, the focusing position of the light beam will satisfy, the focusing position cannot be changed when it is deviated from the intended position. Even if the focusing position can be changed by externally rotating the sheath, the field of view is shifted in response to rotation of the sheath, which makes it impossible to view both a shallower part and a deeper part in the same field of view.
On the other hand, in the optical probe disclosed in International Patent Publication No. WO02/088684, the lens on the leading end portion of the probe is moved in the direction of the optical axis by the wire or the hydraulic pressure to change the distance between the light outlet end of the optical fiber and the lens, thereby changing the magnification (NA, depth of focus) of the lens or to change the distance between the lens and the reflecting mirror, thereby changing the focusing position.
However, in the above structure, since the wire or the hydraulic pipe must be passed through the probe, the space inside the probe is increased, and at the same time, it is necessary to make provision against interference of the fiber and the sheath. Further, since the magnification of the lens and the focusing position are driven by separate drive systems (including wire or hydraulic pressure), a pair of drive systems must be prepared to simultaneously drive the magnification of the lens and the focusing position. To pass a pair of drive systems through a probe to separately drive is more difficult to pass a single drive system through the probe and there is a fear that the drive systems can interfere with each other. Further, when the focal point adjusting system is to be driven to correct the deviation in focusing position generated when the magnification of the lens is changed, the two drive systems must be interlocked. For this purpose, a highly sophisticated control mechanism is required, which adds to the cost. Further, when the drive system is driven to shift the position of the optical fiber in the direction of the axis, the position of the optical fiber is shifted with respect to the optical fiber from the tomography system body. When the optical fiber on the probe side is coupled to the optical fiber on the body side through a direct coupling (ends of the optical fibers are directly mated), the distance between two fibers becomes too large to obtain an excellent coupling unless a mechanism which moves the body side fiber according to the movement of the probe side fiber. Though the change in position can be dealt with insertion of a confocal optical system, this approach requires additional lenses and increases the cost.